The standard method of joining and interconnecting electronic components is soldering with eutectic tin-lead (SnPb) solder in the presence of a fluxing agent. Conventional SnPb solder technology was invented over 5,000 years ago and is increasingly at odds with current manufacturing requirements. The problems with conventional solder technology originate in the high temperatures required, the reflow properties of solder, and the environmental impact of solder and solder fluxes.
Conventional SnPb solder requires temperatures of 200.degree. C. or more for reflow. These temperatures can damage delicate circuit components, and the printed circuit boards (PCBs) on which components are mounted must be made of relatively expensive, heat resistant materials. The high reflow temperatures used can also cause "popcorning", where moisture is explosively released from heated solder and PCBs, creating cracks in standard solder joints. In addition, mounting devices on PCBs with conventional SnPb technology is complicated by the reflow properties of solder. In order to attach components to the bottom side of the PCB, components must be preliminarily attached to the PCB with a dot of adhesive, and the adhesive cured. The adhesive prevents components on the bottom side of a PCB from falling off when components on the top side of the board are soldered in place. Expensive machines called "dot shooters" deposit dots of adhesive, before the components are placed on the PCB.
The environmental problems created by conventional solder technology are attributable to both the toxicity of Pb and the composition and post-soldering treatment of fluxes. Following reflow, conventional solder fluxes must be washed off circuit boards with cleaning fluids, and the cleaning fluids containing flux residues subsequently enter the environment. Previously, the cleaning fluids of choice were chlorofluorocarbons (CFC), which are ozone depleting compounds (ODC). ODCs are currently being phased out in favor of non-ODC cleaners and new, no-clean, low residue solders that do not require cleaning have been developed.
While these developments are positive, they do not eliminate the environmental problems associated with conventional solder technology. For example, ordinary fluxes contain 50 to 65% solvent. These solvents are volatile organic compounds (VOC) which evaporate during the soldering process and enter the atmosphere. No-clean solders use fluxes comprising 20% solids and 80% VOCs. Thus, while no-clean solvents reduce the need for cleaning solvents, they increase the level of pollution attributable to VOCs. In addition, no clean solders typically require inert atmospheres for reflow.
An emerging alternative to standard solder technology uses isotropic electrically conductive adhesives (ICAs) comprising conductive metal particles, often silver, dispersed in a polymerizable matrix. The two component composition of ICAs allows their electrical and mechanical properties to be tailored independently for a particular application. ICAs can be cured at temperatures as low as 130.degree. C. This eliminates damage to heat sensitive components, allows the use of lower cost PCB substrates, and eliminates "popcorning", which does not occur at the lower processing temperatures of ICAs. In addition, the lower processing temperatures of ICAs eliminates the need for dot shooters and the associated manufacturing steps.
Currently available ICAs have found only limited use, because the adhesive-to-metal joint they form is substantially weaker and more susceptible to corrosion than the metallurgical bond created by soldering. ICA joints do not survive a mechanical shock test that is consistently passed by conventional solder joints. In this test, developed by the National Center for Manufacturing Sciences (NCMS) and aptly called the "drop test", a plastic leaded chip carrier (PLCC) having 44 J-shaped leads is mounted to pads on a PCB and dropped from a height of 5 feet six times. None of the currently available ICA joints survive the drop test.
With regard to corrosion, the adhesive-to-metal joints formed by ICAs are susceptible to degradation in elevated humidity/temperature conditions (85.degree. C., 85% relative humidity). Under these conditions, oxygen diffuses into the ICA joints, causing oxidation and corrosion. The resulting ICA joints are unstable and their electrical and mechanical performance is unreliable. The electrical instability may be reduced through the use of oxide penetrating particles, but this does not improve the ICA joint's performance in the drop test.